PVS-Studio warning V610 for Skia

int SkCubicEdge::setCubic(const SkPoint pts[4],
                          const SkIRect* clip, int shift)
{
  ....
  fCurveCount = SkToS8(-1 << shift);
  ....
}

This suspicious code was found in Skia project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '-1' is negative. skedge.cpp 414

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